dt-bindings: iio: adc: stm32-adc: fix id relative path

Fix id relative path that shouldn't contain 'bindings', as pointed out
when submitting st,stm32-dac bindings conversion to json-schema [1].
[1] https://patchwork.ozlabs.org/patch/1257568/

Fixes: a8cf1723 ("dt-bindings: iio: adc: stm32-adc: convert bindings to json-schema")
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/adc/st,stm32-adc.yaml

 # SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
 %YAML 1.2
 ---
-$id: "http://devicetree.org/schemas/bindings/iio/adc/st,stm32-adc.yaml#"
+$id: "http://devicetree.org/schemas/iio/adc/st,stm32-adc.yaml#"
 $schema: "http://devicetree.org/meta-schemas/core.yaml#"
 
 title: STMicroelectronics STM32 ADC bindings